The Skeletal System
Bone Development & Homeostasis
• Objectives: 1) Know the two types of bone
growth
2) Describe the growth of a
long bone
• Agenda: Quick Read
Bone Development Presentation
Reminders: CER writing due today
The Early Embryonic Skeleton
Bone
develops
later →
• Embryonic skeleton – composed of fibrous
connective tissue membranes and hyaline cartilage
Bone Formation and Growth
• Ossification/osteogenesis
• Begins at week 8 of development
• There are two different types of bone
formation
•intramembranous ossification
• endochondral ossification
Intramembranous Ossification
• Results in the formation of cranial
bones and the clavicles.
• All are flat bones
Intramembranous Ossification
• As the matrix forms, trabeculae form, joining
together, forming the lattice of spongy bone
• Outside vascularized connective tissue develops
into the periosteum
• Bone collar of compact bone forms, and red
marrow appears
• Most of this bone will be remodeled into
compact bone over time
Intramembranous Ossification - 1
Intramembranous Ossification -2
Intramembranous Ossification - 3
Intramembranous Ossification - 4
Endochondral Ossification
• Begins in the second month of
development
• Forms all bones below the base of the
skull (Excluding clavicles)
• Uses hyaline cartilage “bones” as models
for bone construction
• Requires breakdown of hyaline cartilage
prior to ossification [Arrange stages]
Endochondral Ossification – 1
• Formation of bone collar
Endochondral Ossification – 2
• Cavitation of the hyaline cartilage
Endochondral Ossification – 3
• Invasion of internal cavities by the periosteal bud, and
spongy bone formation
Endochondral Ossification – 4
• Formation of the medullary cavity; appearance of
secondary ossification centers in the epiphyses
Endochondral Ossification – 5
• Ossification of the epiphyses, with hyaline cartilage
remaining only in the epiphyseal plates and articular
cartilages
Endochondral Ossification
• Vascularized connective tissue develops into the
periosteum outside and the endosteum inside
• Most of this bone will be remodeled repeatedly over
time
Postnatal Bone Growth
Postnatal Bone Growth
• Growth in length of long bones
• Most bone growth stops during
adolescence
• Continued growth of nose and lower jaw
• Is accompanied by remodeling in order to
maintain the proper shape of the epiphysis
and diaphysis
• Cells of the epiphyseal plate proximal to
the resting cartilage form three functionally
different zones: growth, transformation,
and osteogenic
Postnatal Bone
Growth
• Regulated by hGH and the
sex hormones
• In children, cartilage
production continues on the
epiphyseal (distal) side
• cells are destroyed &
replaced to increase the
length of bone
-growth ceases
-bone shows epiphyseal
lines
-clavicle is the last bone to
stop growing
-generally complete by
age 25
-lengthwise growth
completed earlier in
women
Postnatal Long Bone Growth
• Growth in
length of long
bones
• Cartilage on the side
of the epiphyseal plate
closest to the
epiphysis is relatively
inactive
• Cartilage abutting the
shaft of the bone
organizes into a
pattern that allows
fast, efficient growth
Postnatal Long Bone Growth
• Cells in the growth zone
divide quickly, pushing the
epiphysis away from the
diaphysis
• Cells in hypertrophic zone
hypertrophy causing lacunae
to erode and enlarge
• Cartilage matrix calcifies and
the chondrocytes die
• This leaves long spicules of
calcified cartilage at the
epiphysis-diaphysis junction
Postnatal Long Bone Growth
• The spicules become the
osteogenic zone and are
invaded by marrow from
the medullary cavity
• The cartilage is eroded by
osteoclasts and
osteoblasts secrete
matrix to form spongy
bone
• The spicule tips are
removed by osteoclasts
Long Bone Growth
• At the end of adolescence, epiphyseal plates divide
less often and plates are replaced by bone tissue
• Longitudal growth ceases and the
epiphysis/diaphysis fuse.
• Called epiphyseal plate closure
• Females at 18 years
• Males at 21 years
Appositional Bone
Growth
• Growth in width
• From the inside out
• Compact bone lining the
medullary cavity is
destroyed
• Osteoblasts from
periosteum continue to
add more bone to the
outer surface
Long Bone Growth
• Bone diameter can still increase
(appositional)
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter6/animation__bone_growth_in_width.html
Bone Homeostasis - Remodeling
• Remodeling - replacement of old bone by new
• Bone is a very metabolically active tissue
• Spongy transforms to compact or vice versa; old
to new
• Bone is remodeled along the lines of mechanical
stress.
Bone Homeostasis - Remodeling
• Different rates in different
regions
• Distal head of the femur is
replaced ~ every 4 months
• Bone is replaced every 3 to
10 years
Bone Homeostasis - Remodeling
• Delicate balance between breakdown and synthesis
• Too much bone tissue, bones become thick and heavy
• Too much mineral causes bumps or spurs which interfere
with joint function
• Too much Ca2+ loss or crystallization makes bones brittle,
breakable
Bone Resorption
• Accomplished by osteoclasts
• Resorption involves osteoclast secretion of:
• Lysosomal enzymes that digest organic matrix
• Acids that convert calcium salts into soluble forms
• Two control mechanisms
• Stress
• Hormonal
Response to Mechanical Stress
• Wolff’s law – a bone grows or remodels in response
to the forces or demands placed upon it
• Observations supporting Wolff’s law include
• Long bones are thickest midway along the shaft
(where bending stress is greatest)
• Curved bones are thickest where they are most
likely to buckle
Hormonal Mechanism
• Rising blood Ca2+ levels trigger the thyroid to release
calcitonin
• Calcitonin stimulates calcium salt deposit in bone
• Falling blood Ca2+ levels signal the parathyroid
glands to release PTH
• PTH signals osteoclasts to degrade bone matrix and
release Ca2+ into the blood
Hormonal Regulation of Bone
Growth During Youth
• During infancy and childhood, epiphyseal plate activity
is stimulated by growth hormone
• During puberty, testosterone and estrogens:
• Initially promote adolescent growth spurts
• Cause masculinization and feminization of specific
parts of the skeleton
• Later induce epiphyseal plate closure, ending
longitudinal bone growth
Bone Homeostasis - Regulation
• Hormonal regulation of bone growth and remodeling
• hGH (human growth hormone)
• responsible for general growth of all body tissues
• becoming tall or short depends on hGH levels
• works with the sex hormones
• aids in the growth of new bone
• causes degeneration of cartilage cells in epiphyseal
plates
Bone Homeostasis - Regulation
• Hormonal regulation of bone growth and remodeling
• sex hormones – androgens and estrogens - important
for normal bone growth & development
• insulin and thyroid hormones - important for bone and
connective tissue growth & metabolism
Calcium Homeostasis
• Bones are important for Ca2+ homeostasis
• bone tissue is the main reservoir for Ca2+ ions in the
body (500-1000 times more calcium is in bone than in
the rest of the tissues)
• blood levels are regulated very tightly by the endocrine
system
• bone serves as a “buffer” to prevent sudden changes in
blood Ca2+ levels
• too much blood Ca2+ (hypercalcemia) - heart stops
• too little blood Ca2+ (hypocalcemia) - breathing stops
Calcium Homeostasis - Regulation
• 2 hormones are
primarily involved
in Ca2+
homeostasis
• Parathyroid Hormone
(PTH, parathormone)
from the parathyroid
glands increases
blood calcium levels
• Calcitonin from the
thyroid gland
decreases blood
calcium levels